The error implies a client is trying to insert session data, but that machine is not 'known' to the system (i.e. it does not appear in the SignUp computers list). Usually in a situation like this, the problematic client would also be showing as out of order.

You should be able to track this down by looking at a SignUp Server log, as it would have a line like this repeating every minute or so:

Hopefully you have already found the solution as this post is quite old, but I thought I would answer it in case someone else had a similar issue. If you're still seeing the problem, please let us know.